1. Field of the Invention
The present invention relates to polymers having optical and electronic characteristics well-suited for electronic, optoelectronic and photonic applications. In particular, the present invention relates to a composition comprising a .pi.-conjugated polymer; specifically, an exciplex formed from a .pi.-conjugated polymer and another component which, depending upon the circumstances, serves as an electron donor or acceptor. The exciplex of the present invention exhibits enhanced luminescence and photogeneration of charge carriers and excellent quantum efficiency. The present invention is also directed to assemblies and related devices incorporating the exciplex.
2. Background of the Invention
Recent advances in electronics, optoelectronics and photonics have created a need for new materials possessing the requisite optical, electrical and mechanical properties demanded for these applications. A particular class of materials of interest are polymers, which are generally more tractable and easier to process than conventional inorganic semiconductor materials, which characteristics make possible the creation of large and flexible light emitting diodes (LEDs) for displays.
One of the more promising polymer candidates for such uses are the .pi.-conjugated polymers; that is, polymers whose backbone contain single bonds alternating with double bonds (which include those provided by way of rings and aromaticity) or triple bonds. Electrically conducting polymers, including .pi.-conjugated polymers, are known also to form ground state charge transfer (CT) complexes, as described, e.g., by J. E. Frommer and R. R. Chance in Encycl. of Polymer Sci. & Eng., pp. 462-507 (1985) and Wellinghoss et al. in U.S. Pat. No. 4,452,725, which patent discloses iodine and bromine CT complexes of poly(3,6-carbazoles). In these ground state CT complexes, the .pi.-conjugated polymer can act as an electron donor or electron acceptor, and small molecules can act as electron donors or electron acceptors to form CT complexes in the ground state; examples of such include iodine, bromine, AsF.sub.5 and AsF.sub.3. While .pi.-conjugated polymers have been employed in light emitting diodes, photodetectors, solar cells and electrographic photoreceptors, they suffer from having less-than-desirable quantum efficiencies and thus ultimately less-than-desirable electroluminescence device quantum efficiencies, which shortcomings include their inability to efficiently emit spectrally pure, i.e., bright, blue light.
Efforts to increase electroluminescence device quantum efficiency in this regard have been directed to "device engineering" techniques. Endeavors of this sort have focused on, for example, the type of metal electrode used, the thickness of the emitter, the use of charge transport layers to, among other things, improve electron transfer from the electrode, and the use of random copolymers in the device, as described, for example, by Bradley in Adv. Mater. (1992), 4, 756; Burroughes, et al., Nature (1990) 347, 539; Braun, et al., Appl. Phys. Lett. (1991), 58, 1982; Gustafsson, et al., Nature (1992), 357, 477; Burn, et al., Nature (1992), 356, 47; Brown, et al., Chem. Phys. Lett. (1992), 200, 46.
As to the difficulty in efficiently obtaining spectrally pure blue light emission from .pi.-conjugated polymers, the problem with the same is associated with an unusually large apparent Stokes shift between the absorption and emission spectra for .pi.-conjugated polymers and with the inability to control or predict a value for the apparent Stokes shift from molecular structure. Hence, although there are many conjugated polymers with a .pi.-.pi.* transition energy on the order of about 2.8 eV or higher, which value would appear adequate for obtaining blue luminescence, the aforementioned inability to predict the Stokes shift throws off any prediction from the .pi.-.pi.* transition energy. The large Stokes shift in .pi.-conjugated polymers is believed to originate from energy relaxation in the course of excitation energy migration among randomly distributed chromophores of different conjugation lengths.
Blue light emitting conjugated and non-conjugated polymers have been reported, including polymers such as poly(p-phenylene), as reported by Green, et al., Adv. Mater. (1982), 4, 36; poly(alkylfluorene) as reported by Ohmari, et al., Jpn. J. Appl. Phys. (1991), 30, L1941; a polycarbonate derivative, as reported by Hosokawa, et al., Appl. Phys. Lett. (1992), 61, 2503; and a copolymer of poly(p-phenylene vinylene) as reported by Yang, et al., Macromolecules (1993), 26, 1188. These polymers, however, emit blue light with less-than-desirable efficiency and/or less than desirable spectral purity.
While attempts have been made to improve efficiency, these have been, as before stated, directed to device engineering techniques, which are usually hampered by the fact that the .pi.-conjugated polymers have a propensity to form excimers; that is, excited state complexes formed from identical molecules. These complexes are generally stable in the excited state, but are substantially dissociative in the ground state. Excimers are known to emit energy, whereafter they return to a dissociated ground state; however, excimer emission from .pi.-conjugated polymers gives rise to weak and inefficient luminescence because, in part, they tend to emit energy in the form of heat rather than luminescence, upon decay. For .pi.-conjugated polymers, studies of excimer-like emissions have been made of poly(pyridine-2,5-diyl) by Yamamoto, et al., J. Chem Soc. Chem. Commun. (1990), 1306.
In addition to excimer emissions of .pi.-conjugated polymers, studies of photo-induced electron transfer between poly[2-methoxy,5-(2'-ethyl-hexyloxy)-p-phenylene vinylene (MEH-PPV) and C.sub.60, have been reported, Scaricifti, et al., Science, 258, 1474 (27 Nov. 1992), the result here being ionization, i.e., the generation of ion radicals from the separation of charges, which does not result in highly efficient luminescence.
Thus the art recognizes a continuing need to improve quantum efficiency of .pi.-conjugated polymers in regard to luminescence and photogeneration of charge carriers, including the pressing need for efficient generation of spectrally pure blue light.